Recently, an apparently new swine disease causing heavy losses in breeding herds has appeared in the United States and in certain regions of Canada. Keffaber, K. K., "Reproducave Failure of Unknown Etiology", American Association of Swine Practitioners Newsletter, 1:109 (1989). A similar disease has also been reported in certain regions of Germany, in the Netherlands and also in the United Kingdom and Spain. Wensvoort, G. et al., "Blue Ear Disease of Pigs", Vet. Rec., 128:574 (1991). The most prominent clinical symptom of the disease is the farrowing of dead, or sickly piglets, with some healthy-appearing piglets subsequently faring poorly or developing impaired breathing, CNS symptoms and dying. In some affected herds, up to seventy-five percent of all piglets may be lost. The economic consequences of the disease, accordingly, are devastating. It has been called "mystery swine disease", swine infertility and respiratory syndrome (SIRS), "blue ear disease", and porcine reproductive and respiratory syndrome.
SIRS is an apparently infectious disease, characterized by reproductive failure, respiratory disease and variable clinical signs including anorexia, fever, dyspnea and mild neurological signs. The disease affects all types of swine production facilities, and may be among the most costly of diseases affecting the swine industry today. Polson, D. P. et al. "Financial Implications of Mystery Swine Disease", Proceedings, Livestock Conservation Institute, Denver, Colo. (Oct. 6, 1990).
The infection of sows may go unnoticed, or may manifest itself by impaired general condition lasting one or a few days. For example, the sows may go off feed and show body temperatures above or below normal. In the farrowing phase, the signs of disease include depression, lethargy, pyrexia, occasional vomiting, abortions, stillborns and/or delivery of mummified fetuses. The most frequent clinical sign is the dramatic increase in the number of stillborn pigs. The number of stillbirths can be as high as twenty percent to thirty percent of all births in an infected herd. Many of the fetuses delivered as stillbirths will be macerated, appearing to have been dead for 24 to 48 hours.
As stated above, a major component of SIRS is reproductive failure which manifests itself as premature births, late term abortions, pigs born weak, increased stillbirths, mummified fetuses, decreased farrowing rates and delayed return to estrus. Such clinical symptoms will typically be observed in a herd from 4 to 16 weeks, or even longer. Stillborn fetuses in affected litters often are in the early stages of mummification, as evidenced by tan-brown discoloration of the skin and post-mortem autolysis. Dome-shaped malformation of fetal skulls is sometimes seen.
Clinical signs of respiratory disease are most pronounced in piglets under 3 weeks of age, but are reported to occur in pigs of all ages in infected herds. The diseased piglets grow slowly, have toughened hair coats, respiratory distress ("thumping") and increased mortality (up to about eighty percent pre-weaning mortality).
Findings in preliminary studies of gross and microscopic lesions of SIRS diseased swine suggest that microscopic lung lesions are an important clinical feature of this disease. Despite pronounced respiratory signs, lungs uncomplicated by secondary bacterial infection are either grossly normal or have a mild, diffused tan-gray discoloration of lung surface. Microscopic examination of lung tissue of SIRS-diseased piglets, however, reveals a characteristic pattern of interstitial pneumonitis. Collins, J. E. et al. "Respiratory Disease in a Swine Herd Experiencing a Reproductive Failure Syndrome", Proceedings, Minnesota Swine Conference for Veterinarians, p. 254, St. Paul, Minn. (Sep. 10-18, 1990).
The incidence of SIRS or "mystery swine disease" is widespread in the United States, reported in at least eleven states. The primary causative agents being studied, as reported in the published literature, include encephalomyocarditis virus (EMC), swine influenza virus (SIV), mycotoxins and chlamydia.